[Gmecm] Vacuum referenced TBI fuel pressure regulator

[David Haggard]

  > Since it has come up recently, how many have tried modifying their 
  > TBI's fuel pressure regulator to be vacuum-referenced instead of a 
  > fixed pressure?  What (if anything) did it do for you?

  > Since there's no pressure changes at the point of injection 
  > (unlike PFI) reducing fuel pressure under light loads is 
  > unnecessary. If you need to raise your fuel pressure you can 
  > remove the regulator, grind off the brazing, and adjust the 
  > threaded factory adjustment.

On my 305, I'm using a Mallory external, vacuum-modulated pressure regulator, mounted on the outlet side of a 2" bore TBI from a '93 big block 454 Chevy pickup engine. I left the OEM regulator in place, set for 12 psi, but with the Mallory in the line downstream from the OEM regulator, it's the Mallory that controls the pressure at the injectors. I left the OEM regulator in place as a sort of "limp home" backup.

I'll tell you all the story of why I put in the Mallory, and you can decide for yourselves whether I'm crazy or on the right track.

I knew I needed larger-than-stock injectors for the power I wanted to make. 40 lb/hr injectors at 12 psi weren't going to feed between 360 and 375 horses, which was the power range I was shooting for. For me, it wasn't pressure at the point of injection I cared about. It was delivery volume. I kept reading about how "TBI can't make serious horsepower because it can't deliver enough fuel," so I figured I had to up the volume big time.

The engine seemed to like 15 psi for best idle, regardless of injector size. I think it's something to do with the small displacement combined with the big throttle bore. I may be wrong, but I believe the extra pressure helps with atomization at idle. Just a theory. No proof except that the engine idles best with 15 psi.

The Mallory regulator was the only one I found on the market that would give me adjustability from 12 to 30 psi, plus directly mount a pressure gauge. It can be used either with or without vacuum modulation. With vacuum, the pressure increases 1 psi for every 2 inches of vacuum drop. It's easy to adjust and works very consistently.

To get the pressure I wanted, and hoping it could supply the volume I needed without breaking my bank account, I opted for a 1990 Camaro TPI fuel pump. It was a direct replacement for my original in-tank TBI pump, and would put out 45 psi at any flow I thought the engine would want.

I chose the "Yellow/Brown" 5.7 litre "cop car" injectors, which are rated 65 lb/hr at 12 psi.

I experimented with fixed fuel pressures from 12 psi up to 28 psi, both before and after getting a flow-matched set of the "cop car" injectors. The bigger injectors turned out to be the better choice. The OEM injectors couldn't put out enough fuel even at 28 psi, which I set as my limit because I had read that TBI injectors can tolerate "up to" 30 psi without leakage. 

With the larger injectors, I could either cut back the fuel pressure to 15 psi and get a good idle, or up the pressure to above 20 psi so it wouldn't go lean at WOT. I didn't seem to be able to find middle ground with fixed pressure.

So I put the vacuum hose on the regulator, and set it for 15 psi at idle. Then I did a lot of fuel table tuning with WinALDL readings, and then started adjusting the WOT AFR table using a stopwatch.

My engine idles at about 15 inches of vacuum at 700 rpm. Forcing the ECM into open loop at idle gives me a smooth idle with some lope. I'm still working on closed loop idle settings that will give me the same result, but it's been a struggle. Anyway, manifold vacuum at the base of the TBI is as close to 0 at WOT as my vacuum gauge will register. So at WOT, the injectors are getting about 22 psi.

Since injector volume is roughly proportional to fuel pressure, the injectors act like 80 lb/hr units at idle with 15 psi. But at WOT, they supposedly become about 120 lb/hr. (It's probably not that linear, but that's the best calculation I can come up with.) Because of this, I have to "lie" to the ECM in the "Power Enrichment AFR vs. RPM" table (the "WOT AFR" table).

I totally flattened Fuel Table #2 with a VE setting of 25, and tuned Fuel table #1 to get cruise/idle BLM's as close to 128 across the board as possible. With that, my WOT AFR table is this--
	RPM	AFR
	0	13.0
	400	13.0
	800	13.0
	1200	13.2
	1600	13.4
	2000	13.5
	2400	13.7
	2800	13.9
	3200	14.0

Actual AFR at WOT holds in the area of 12.5:1 with these settings.

The result is that my AFR is completely under control at all engine speeds and loads, where without the modulated pressure I was either too rich at idle or too lean at WOT. It also turns out that when my next engine is done (a 420 horse 350 or 383), about all I'll have to do is raise the BPW constant for the increased displacement. Of course, it won't be that simple. I'll have to go through the WinALDL/stopwatch routine again.

Anyway, you all can decide I'm nuts with this setup. But I don't care 'cuz hey, it works. The vacuum-modulated fuel pressure did add another 2 or 3 dozen chips that I had to burn in the tuning process (I don't have a ROMulator.), but it definitely solved my mixture/deliver issues.

David Haggard

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